U.S. Pat. No. 3,087,807 discloses a process with which it is possible to produce a porous metal article of any geometric shape. This entails a mixture of a metal powder and a blowing agent being, in the first step, compacted cold under a pressure of at least 80 MPa. The mixture is subsequently extruded in order to be at least 87.5% compacted. This high degree of compaction is regarded as necessary to destroy the oxide coating on the particles by the mutual friction thereof during the compaction process and to bond the metal particles together. The extruded rod produced in this way can be foamed to a porous metal article by heating to at least the melting point of the metal. The foaming can take place in various molds so that the finished porous metal article has the required shape. The disadvantages of this process are that, because of its two-stage compaction operation and the very high degree of compaction required, it is elaborate and restricted to semifinished products which can be produced by extrusion. The only blowing agents which can be used in the process disclosed herein are those with a decomposition temperature above the compacting temperature, because otherwise the gas would escape during the extrusion operation. The extrusion operation in the process described herein is regarded as necessary because the bonding of the metal particles is produced by the high temperatures occurring during the extrusion operation and by the mutual friction of the particles, i.e. by fusion of the particles together.
EP-B-0 460 392 describes a process for producing foamable metal articles with predominantly closed porosity, where the resulting pores are intended to be uniformly distributed in the entire metal article and to have a uniform size, wherein a mixture of at least one metal powder and at least one gas-producing blowing agent powder is produced and compacted to a semifinished product. The process described herein is characterized in that the hot compaction takes place at a temperature above the decomposition temperature of the blowing agent, with the bonding of the metal powder particles taking place mainly through diffusion, and under a pressure which is high enough to prevent decomposition of the blowing agent in such a way that the metal particles are firmly bonded to one another and represent a gas-tight seal for the gas particles of the blowing agent.
DE-C-41 24 591 describes a process for producing foamable metal articles by rolling a powder mixture, wherein the powder mixture consists of at least one powder containing a blowing agent and one metal powder, wherein the powder mixture is packed into a metal hollow section and rolled. The cold press article obtainable according to this publication can not only be heated before the rolling operation but also be reheated after the individual rolling stages. In this case too, the cold press article is heated to a temperature above the decomposition temperature of the blowing agent.
DE-A-44 26 627 relates to a metallic composite material and to a process for its production. The metallic composite material having a core of one or more porous metallic materials and at least one outer layer of solid material has metallic linkages between the core and the outer layer/outer layers.